Biochemical studies of the estrogen forming enzyme (aromatase) of human placenta and ovary are being carried out. Studies of nonendocrine tissues such as fetal tissue and tumors containing aromatase are aimed at defining their paraendocrine function with respect to steroid hormone production. Physiologic control mechanisms for estrogen formation in the normal ovary are under investigation. The possible role of placental steroids in preventing fetal allograft rejection during pregnancy is being explored.